i) Field of the Invention
The present invention relates to a method for reducing degradation of imaging performance by maneuver of a radar platform in a synthetic aperture radar and a method of measuring or determining a position of the moving platform by using the radar.
ii) Description of the Related Arts
In FIG. 6, there is shown a conventional synthetic aperture radar, as disclosed in "Motion Compensation For Synthetic Aperture Radar" by J. C. Kirk, Jr., IEEE. Transaction on Aerospace and Electronic Systems, Vol. AES-11, No. 3, pp. 338-348. In FIG. 6, the synthetic aperture radar is comprised of a transmitter 1, a T/R (transmitter/receiver) changeover circulator 2, a T/R antenna 3, a receiver 4, a local oscillator 5 for supplying a local oscillation signal to the transmitter 1 and the receiver 4, a reference signal generator 6 for phase compensation, a complex multiplier 7 for carrying out a complex multiplication of two output signals supplied from the reference signal generator and the receiver 4, an image reproducer 8 connected to the complex multiplier 7, a display 9 connected to the image reproducer 8, an inertial navigation system 10 for supplying data such as a position, a velocity and the like of a radar platform to the reference signal generator 6, a clutter tracker 11 for detecting a central spectrum of an output signal of the complex multiplier 7 to supply the detected central spectrum to the reference signal generator 6, and an antenna direction controller 12 for directing a beam of the antenna 3 to an object to be acquired according to a reference signal for phase compensation, supplied from the reference signal generator 6.
FIG. 7 illustrates a conventional motion compensation method of the synthetic aperture radar shown in FIG. 6. In FIG. 7, numerals 13, 14 and 15 denote the object to be acquired, a reference point for motion compensation, and the radar platform carrying the synthetic aperture radar, respectively.
Next, the operation of the synthetic aperture radar described above will now be described in connection with FIGS. 6 and 7.
A radio frequency signal generated by the transmitter 1 is irradiated toward the object 13 to be acquired from the T/R antenna 3 via the T/R changeover circulator 2. The radio frequency signal reflected by the object 13 is received by the T/R antenna 3 and the received radio frequency signal is sent to the receiver 4 via the T/R changeover circulator 2 and is amplified and detected therein. The detected signal is supplied to the complex multiplier 7 and the complex multiplier 7 carries out a complex multiplication of the detected signal and the reference signal for motion compensation generated by the reference signal generator 6. Hence, the same radar echo is obtained as the radar platform 15 is stabilized against the maneuver. At this time, a phase shift amount to be given to the reference signal for motion compensation can be calculated from a relative distance between the reference point 14 for phase compensation as any point within the object 13 and the radar platform 15, as shown in FIG. 7. The position of the radar platform 15 is obtained from the inertial navigation system 10.
The position determining precision of the radar platform 15, required in the calculation of the phase shift amount is considered to be approximately a transmitting wavelength, and in case of the synthetic aperture radar mounted on an aircraft and a space vehicle, its wavelength becomes approximately 3 cm to 10 cm. In general, the position determining precision by using the inertial navigation system 10 is approximately several meters to several tens of meters. Thus, the position determining precision by the inertial navigator 10 is bad and it is necessary to compensate for a difference from the above-described necessary precision (approximately 3 cm to 10 cm). Hence, first, the output signal of the complex multiplier 7 is taken into the clutter tracker 11 for detecting the central frequency of its spectrum. Then, the phase compensation amount is calculated so that the difference between the obtained central frequency and a central frequency of a received signal spectrum to be obtained when there is no maneuver of the radar platform 15. In the reference signal generator 6 for phase compensation, by adding the calculated phase compensation amount to the phase shift amount calculated from the relative distance as described above, the insufficient phase measuring precision of the inertial navigation system 10 is compensated. At this time, the measuring precision of the central frequency of the received signal spectrum depends on a signal to noise power ratio and a variation of a radar cross section of the object 13 to be acquired with the motion of the radar platform 15.
Further, as a position determining method in place of the inertial navigation system 10, a GPS (global positioning system) is well-known, in which high frequency signals are irradiated from transmitters mounted on at least four satellites launched into mutually different orbits and the irradiated high frequency signals are received by a receiver mounted on a radar platform to measure a position of the receiver. As examples of the GPS, a NAVSTAR/GPS and a GLONASS are known. However, in this system, different oscillators (clocks) are used in the transmitter and receiver sides and hence information relating to the phases of the signals used in the transmitter side can not be utilized in the receiver side. Hence, since the relative distances between the transmitters and the receiver are calculated from the signal transmission times between the same, the precision becomes approximately several meter at the most and the position determining precision is insufficient for use in the stabilization of the synthetic aperture radar. Further, in the GPS, it is insufficient to use three transmitters for determining the position of the radar platform in the three-dimensional space. That is, the clock mounted to each transmitter has a time offset and thus this time offset must be treated as an unknown quantity. Hence, in order to solve four-dimensional simultaneous equations, a fourth transmitter (a fourth satellite) is required.
In the conventional synthetic aperture radar, since the stabilization is carried out as described above, with the motion of the radar platform, the radar cross section of the object to be acquired is varied, or by a receiver noise, an error is caused in measuring the frequency of the received signal. These are regarded as the maneuver of the radar platform and hence the phase compensation amount including the error can be calculated.
Further, considering as the position determining method of the radar platform, the determining precision by the inertial navigation system is approximately several tens of meters and it becomes approximately several meters even in the GPS. Hence, a position determining method with a higher precision than the method utilizing the signal transmission time between the transmitter and the receiver has been required.